1. Technical Field
The present invention relates to a delay-line demodulator to be used for an optical fiber communication. In particular, it relates to a delay-line demodulator comprising a Mach-Zehnder interferometer (MZI) of a planar lightwave circuit (PLC) type for demodulating a signal of such as a differential quadrature phase shift keying (DQPSK), a differential phase shift keying (DPSK), or the like, regarding the optical fiber communication using a transmission method of a dense wavelength division multiplexing (DWDM).
2. Background Art
In recent years, an examination for a speed up of an optical transmission system (to be a line speed of 40 Gbps) is actively performed against a backdrop of a rapid spread of a broadband communication. However, there is provided a problem that a quality of a communication line cannot help but become poor because of a deterioration of a signal waveform due to a decrease of a duration per one bit of a light signal affecting a characteristic of an optical fiber in a case of speeding up the line speed. Moreover, it becomes hard to construct a network system with using a conventional fiber network because a repeater is required for such as converting a light signal into an electric signal during passing a transmission path and reconverting into a light signal therefrom, for performing a long distance transmission of the 40 Gbps level.
Therefore, there are investigated and developed currently, such as the differential quadrature phase shift keying (DQPSK) method, the differential phase shift keying (DPSK) method, or the like, that are a multivalued phase shift keying method to be able to reduce the deterioration of the signal waveform by being widened the duration of a light signal.
The DQPSK transmits four data with corresponding to four different light phase differences. That is to say, it is a phase shift keying method to change a phase of a propagation wave (, +/2, +, +3/2) for the four data as values (0, 1, 2, 3) of individual symbols comprised of data of two bits, with corresponding to a variation in values of neighboring two symbols. And then according to a 40 Gbps DQPSK communication method using such the DQPSK method, it becomes able to transmit a distance as four times longer than that for a 40 Gbps transmission using a conventional binary phase shift keying method. Moreover, it is considered that it becomes possible to construct the network system among large cities with using the conventional fiber network, according to such the DQPSK method.
While, regarding such the DPSK, a phase of a propagation wave (or +) is determined with corresponding to a variation in (or difference of) between a value of a bit (zero or one) transmitted previously and a value of a bit (zero or one) to be transmitted next.
So far, as a delay-line demodulator for demodulating such the DQPSK signal or the DPSK signal at a receiving device, there is known such as an optical receiving circuit as disclosed in a patent document 1, a demodulator as disclosed in a patent document 2, or the like.
Here, the optical receiving circuit as disclosed in the patent document 1 comprises an one symbol delay element at one side of one pair of optical paths, and a interferometer (the Mach-Zehnder interferometer) for propagating an input RZ-DPSK signal, that is to say, a DPSK signal to be performed a return to zero (RZ) phase shift keying therefor, via such the one pair of the optical paths. Moreover, by using such the interferometer (an one bit delay interferometer), the input RZ-DPSK signal becomes to be transformed into one pair of intensity modulated light signals as complementary therebetween.
Further, according to the patent document 2, there is disclosed a demodulator for demodulating using a Michelson interferometer regarding a light signal performed the differential phase shift keying (DPSK), or a light signal performed the differential quadrature phase shift keying (DQPSK).
[Patent Document 1] Japanese Patent Application Publication No. 2007-060442
[Patent Document 2] Japanese Patent Application Publication No. 2007-151026